The present invention relates to an apparatus for reading a hologram, in particular a computer-generated hologram, having a radiation source for irradiation of the hologram with a read beam composed of electromagnetic radiation, and having an outlet opening for recording an image which is produced from the hologram.
In general, a wide range of readers for holograms are known from the prior art. Many of these readers relate to conventionally illuminated holograms, although some also deal with computer-generated holograms (CGH).
Computer-generated holograms, in particular digital holograms, are two-dimensional dot matrices, preferably with very small dimensions, which comprise individual dots with different optical characteristics.
Images and/or data are/is reproduced by illumination of a computer-generated hologram with, for example, a coherent electromagnetic wave, in particular a light wave, by diffraction in transmission or reflection. The different optical characteristics of the individual dots may be reflection characteristics, for example by means of surface topography, varying optical path lengths in the material of the storage medium (refraction indices) or color values of the material.
In contrast to conventional holograms, the diffraction structures on the hologram plane are not illuminated conventionally but are calculated by computers using algorithms and are introduced point-by-point into the material of the storage medium. These are so-called computer-generated holograms (CGH), which can be used for classification, for identification and for individualization of any given objects. In this case, they are increasingly being used in safety-relevant areas.
The computer-generated holograms explained above may carry different information items. On the one hand, the hologram may carry holographic information which reproduces a directly legible information item (script, figure, graphics) during reading, i.e. on reproduction. The information that is read may likewise be scrambled, in particular digitized. By way of example, the reproduced hologram then contains a digital matrix similar to a one-dimensional or two-dimensional barcode.
In addition, the dot distribution of the computer-generated hologram may in its own right have directly legible information with very small dimensions, for example a microscript or a microimage. The directly legible information must be enlarged for identification, that is to say by a process which differs from the reading of the hologram. The microscript and/or the microimage may itself in turn be scrambled. For example, a microimage may itself represent digital information of a one-dimensional or two-dimensional barcode. It is thus possible to combine holographic and directly legible information in the holograms to be read.
Particularly in the case of Fourier transformation holograms, irrespective of whether these are conventionally illuminated or computer-generated holograms, the problem exists that a transformation lens is required for reconstruction of the stored information. In the traditional case, this is placed in the beam path, behind the hologram, at the same distance as its positive focal length from the hologram. The reconstructed image can then be observed clearly on the rear focal plane of the lens. Alternatively, the transformation lens can also be positioned in direct contact with the hologram plane, or even in front of the hologram. The reconstructed s image can once again be observed clearly on the rear focal plane of the transformation lens.
Alternatively, it is also possible to project the reconstruction onto a screen, which is positioned comparatively far away. This is associated with the fact that the far-field diffraction pattern corresponds to a Fourier transformation. However, this option is associated with less clarity, less brightness and often with excessively large dimensions of the image and, for these reasons, is not preferred. It is also known for a CCD detector (charge coupled device) to be used as an alternative to a screen for the reconstruction. Solutions such as these are, however, costly and complex. By way of example, reference is made to DE 42 37 415, EP 0 802 462 or U.S. Pat. No. 5,623,347 with regard to the prior art which discloses such known apparatuses.
WO 02/084588, on which the present invention is based, discloses the use of the human eye itself as a Fourier transformation lens, in order to simplify a reader for reading holograms. This makes it possible to considerably simplify apparatuses for reading holograms.